Immunoglobulin-like transcript (ILT) 3 is a cell surface molecule that is a member of the immunoglobulin superfamily. ILT3 is selectively expressed by myeloid antigen presenting cells (APCs) such as monocytes, macrophages, and dendritic cells (DC). The cytoplasmic region of ILT3 contains putative immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Co-ligation of ILT3 to stimulatory receptors expressed by APCs results in a blunting of the increased [Ca2+] flux and tyrosine phosphorylation triggered by these receptors. Signal extinction involves SH2-containing protein tyrosine phosphatase 1, which is recruited by ILT3 upon cross-linking. ILT3 can also function in antigen capture and presentation. It is efficiently internalized upon cross-linking and delivers its ligand to an intracellular compartment where it is processed and presented to T cells (Cella, et al. (1997) J. Exp. Med. 185:1743-1751).
Thus, ILT3 is an inhibitory receptor that can negatively regulate activation of APCs and can be used by APCs for antigen uptake. The development of agents useful in modulating signaling via ILT3 would be of great benefit in modulating immune responses.